1. Field of the Invention
This invention relates to an aqueous suspension suitable for use as a color-developing agent, more specifically, to an aqueous suspension of a multivalent-metal-modified salicylic acid resin having a novel composition and disclosed in any one of the present assignee's or applicant's preceding patent applications to be referred to subsequently, and notably, to an aqueous suspension of a novel oil-soluble multivalent-metal-modified salicylic acid resin, which is useful as a material for pressure-sensitive copying papers. This invention is also concerned with a method for the preparation of the above-mentioned aqueous suspension.
2. Description of the Related Art
A pressure-sensitive copying paper is generally composed of a sheet (CB-sheet) coated with microcapsules of a non-volatile organic solvent containing an electron-donating organic compound (so-called pressure-sensitive dyestuff) and another sheet (CF-sheet) coated with an aqueous coating formulation containing an electron-attracting color-developing agent. The CB-sheet and CF-sheet are arranged with their coated sides maintained in a contiguous relation. The microcapsules are ruptured, for example, by a writing or printing impression of a ballpoint pen or a typewriter, whereby the solution of the pressure-sensitive dyestuff is caused to flow out of the capsules and is then brought into contact with the color-developing agent and a color is hence produced. By changing the combination of the layer of the microcapsules containing the pressure-sensitive dyestuff and the layer of the color-developing agent, many copies can be produced and self-contained pressure-sensitive copying papers (SC paper sheets) can be produced.
As a colorless or slightly-colored dyestuff precursor useful in such pressure-sensitive copying papers, one or more compounds are selected from:
(1) triarylmethanephthalide compounds such as crystal violet lactone; PA0 (2) fluorane compounds such as 3-dibutylamino-6-methyl-7-anilinofluorane; PA0 (3) pyridylphthalide compounds; PA0 (4) phenothiazine compounds; and PA0 (5) leucoauramine compounds.
Said one or more compounds are dissolved in a hydrophobic phobic high boiling-point solvent and microencapsulated for their application.
As electron-attracting color-developing agents on the other hand, there have conventionally been used (1) inorganic solid acids such as acid clay and activated clay, (2) oil-soluble phenol-formaldehyde condensation products and their multivalent-metal-modified products and (3) multivalent metal salts of substituted salicylic acids by way of example. These color-developing agents cannot however provide marks having sufficient stability, so that produced color marks may be discolored or faded during their storage or their water resistance or solvent resistance may be insufficient.
As color-developing agents free of such problems, the present inventors have already found novel multivalent-metal-modified salicylic acids on which patent applications have been made [Japanese Patent Application No. 262019/1986 and others, which are the priority applications for U.S. patent application Ser. No. 111,763, filed on Oct. 23, 1987, and Japanese Patent Application No. 87030/1987].
In order to produce pressure-sensitive copying papers by using a color-developing agent, the color-developing agent is generally wet-ground in the presence of a surfactant into an aqueous suspension of fine particles having a particle size of 1-10 .mu.m. A dispersant is used for this purpose.
The selection of the combination of particles to be dispersed and a dispersant for obtaining a good dispersion system is however based primarily on experiences, and there is no general rule for the selection. Upon selection of a dispersant, it is necessary to take into consideration not only dispersing effects but also influence and the like to the action of particles to be dispersed.
For these reasons, it is not easy to combine one of the multivalent-metal-modified salicylic acid resins with its matching dispersant to prepare an aqueous suspension having good properties in various aspects such as the state of dispersion, stability, color-developing ability, etc. Anionic high-molecular surfactants of the polycarboxylic acid type, specifically, the sodium salts of maleic anhydridediisobutyrene copolymers are generally employed as dispersants for p-phenylphenol-formaldehyde and p-octylphenol-formaldehyde polymers which are currently used as color-developing agents for pressure-sensitive recording paper sheets. When each of these surfactants is used as a dispersant for the conversion of any one of the multivalent-metal-modified salicylic acid resins into an aqueous suspension, the formation of an inconvenient complex salt takes place between the multivalent metal and the carboxylic acid salt. Hence, the dispersing effects and dispersion stability are reduced, hardly defoamable foams are formed, and the physical properties of the color-developing agent are changed due to modification of the multivalent-metal-modified salicylic acid resin as a dispersed substance. It is by no means possible to obtain any suspension which may be used practically.
Although some of salts of formaldehyde condensation products of naphthalenesulfonic acid, salts of ligninsulfonic acid and like salts, which were used previously for color-developing agents of the phenol-formaldehyde condensation products, have dispersing effects for the multivalent-metal-modified salicylic acid resins, they substantially lack practical utility because paper surfaces are colored or undergo light-yellowing due to the inclusion of the dispersant when employed in pressure-sensitive copying papers.